Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a radiation-sensitive material (such as, for example, photoresist) associated with a semiconductor wafer. The radiation-patterning tool can be referred to as a photomask or a reticle. The term “photomask” traditionally is understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized with their traditional meanings.
Radiation-patterning tools contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. As discussed previously, the wafer is provided with a layer of radiation-sensitive material (such as, for example, photosensitive resist material, which is commonly referred to as photoresist). Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers a pattern defined by the radiation-patterning tool onto the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and a decrease in the dimensions of conductor elements which connect those integrated circuit devices. The demand for ever smaller integrated circuit devices brings with it demands for ever-decreasing dimensions of structural elements, and ever-increasing requirements for precision and accuracy in radiation patterning.
FIG. 1 shows a flow chart illustrating a typical process utilized for creating a pattern for a radiation-patterning tool. At an initial step 10, a preliminary design is created for the radiation-patterning tool and verified. The creation of the design begins with provision of a desired pattern which is ultimately to be formed in photoresist. Subsequently, elements are developed for the radiation-patterning tool to roughly produce the desired pattern on photoresist from radiation passed through the radiation-patterning tool. The elements form a rough correspondence to the desired pattern in that the first approximation of the elements largely ignores effects of interference on radiation passing through the radiation-patterning tool.
After the design is believed to be complete, (i.e., once it is believed that all patterned features which are to be patterned in photoresist with the radiation-patterning tool are represented by elements in the design) the design is submitted to a verification process to confirm that the design is complete.
After the design has been created and verified, it is subjected to optical proximity correction (shown as step 20 in FIG. 1). The optical proximity correction takes into account various interference factors that influence radiation passing through a radiation-patterning tool (i.e., constructive and destructive interference effects that result from passing radiation through patterns having dimensions on the same order as the wavelength of the radiation, or smaller). The optical proximity correction can be utilized to correct all parts of the design, or only some parts of the design. In other words, the optical proximity correction can be applied to only some portions of a design, while other portions are not optical proximity corrected. Typically there will be a verification step following the optical proximity correction.
The steps of generating a design from a desired pattern which is to be provided in photoresist, verification of the design, optical proximity correction, and verification of the correction, are typically accomplished primarily through the use of software. A suitable software package which can be utilized for one or more of the steps is HERCULES™/TAURUS OPC™, which is available from Synopsys Corporation™.
The optical proximity correction creates a dataset which is subsequently translated into a pattern formed on a radiation-patterning tool. The process of translating the dataset into a pattern on the radiation-patterning tool is frequently referred to as taping the pattern onto the radiation-patterning tool. In such context, the terms “tape” and “tape out” refer to a process of transferring the dataset to appropriate hardware which writes a pattern represented by the dataset onto the radiation-patterning tool. The process of writing onto the radiation-patterning tool can be accomplished by, for example, laser writing and/or electron-beam writing methodologies. The step of taping the pattern onto the radiation-patterning tool is shown in FIG. 1 as step 30.
After the pattern has been formed on the radiation-patterning tool, the tool can be utilized for patterning radiation in semiconductor fabrication processes. FIG. 2 illustrates an exemplary apparatus 40 in which a radiation-patterning tool is utilized for patterning radiation. Apparatus 40 comprises a lamp 42 which generates radiation 44. Apparatus 40 further comprises a radiation-patterning tool 46 through which radiation 44 is passed. A semiconductor substrate 48 having a radiation-sensitive material 50 thereover is illustrated associated with apparatus 40. The radiation passing through radiation-patterning tool 46 impacts radiation-sensitive material 50 to form a pattern within the radiation-sensitive material. The process of forming a pattern in a radiation-sensitive material with a radiation-patterning tool can be referred to as a printing operation.
Radiation-patterning tool 46 typically comprises an opaque material (such as chrome) over a transparent material (such as a glass). Radiation-patterning tool 46 has a front side where the pattern is formed as features (or windows) extending through the opaque material, and has a back side in opposing relation to the front side. The shown radiation-patterning tool has two opposing sides 45 and 47, and in practice one of the two sides would be the front side (typically side 45) and the other would be the back side. In some applications features can be printed on both the front side and back side of the radiation-patterning tool.
As discussed above, radiation-patterning tool 46 will typically have a pattern with dimensions on the order of the wavelength of the radiation passing through the radiation-patterning tool, or smaller. Accordingly, various interference effects can occur as the radiation passes through the radiation-patterning tool so that the radiation exiting the radiation-patterning tool will transfer a pattern somewhat different than the pattern of the radiation-patterning tool. Such is illustrated diagrammatically in FIG. 3. Specifically, FIG. 3 illustrates an exemplary pattern 60, which can be desired to be formed in a radiation-sensitive material, and illustrates an approximation of a pattern 70 which would be formed in a radiation-patterning tool to generate the pattern 60. Pattern 70 is referred to as an approximation because the pattern is a qualitative representation of the type of pattern utilized in the radiation-patterning tool for generating pattern 60, rather than a quantitative representation.
The FIG. 1 process can, for example, start with a pattern identical to pattern 60 being provided at the design step (10) of the radiation-patterning tool fabrication process, and such design would then be converted to the shape 70 during the optical proximity correction (20) step.
FIG. 4 illustrates an exemplary design which can be desired to be formed in a radiation-sensitive material 80, and illustrates elements in a radiation-patterning tool 84 utilized to create such design.
Radiation-sensitive material 80 is illustrated in top view, and the design formed within the material comprises a plurality of repeating units 82. The shown units 82 are circular in patterned dimension, and can be utilized, for example, in forming contact openings. One of the shown units is centered around a location 83, and another of the units is centered around a location 85.
Radiation-patterning tool 84 comprises a plurality of repeating elements 86. The elements 86 are in a one-to-one correspondence with the units 82 formed in the radiation-sensitive material. Further, each of elements 86 is approximately square in shape. In operation, radiation is passed through radiation-patterning tool 84 to form the pattern of printed images 82 on radiation-sensitive material 80. Regions 86 of the radiation-patterning tool can be either more transparent to radiation than surrounding regions of the radiation-patterning tool, or can be less transparent, depending on whether the radiation-sensitive material corresponds to a positive or negative material. If elements 86 are more transmissive to radiation than surrounding regions, the elements 86 can effectively be windows which allow radiation to pass through those specific regions of the radiation-patterning tool.
In particular aspects of the prior art, printed images 82 correspond to regions where light has passed through windows 86 of radiation-patterning tool 84. If windows 86 have dimensions on the order of the wavelength of the light passing through the windows, there can be sidelobes of energy 88 extending around each of features 82. The sidelobes are commonly referred to as sombreros. The energy within the sombreros is generally too low to form a printed feature within the radiation-sensitive material 80. However, if two or more sidelobes converge near one another, they can form a printed feature at the location of the convergence. Regions 90 and 92 illustrate locations where four sidelobes converge near one another, and accordingly illustrate locations where printed features can undesirably occur due to the convergence of the sidelobes.
Sidelobe convergence is described in additional detail with reference to FIGS. 5-7. Referring initially to FIG. 5, electric field strength of light passing through a window 86 to form a feature 82 centered around location 83 is illustrated (the figure is for diagrammatic purposes only, and not quantitative). A large positive field strength occurs at location 83, and negative sidelobes occur at locations 88. The large positive field strength can be referred to as a primary lobe.
The effect induced by radiation impacting radiation-sensitive material 80 is actually proportional to the intensity of the radiation, rather than the field strength, and the intensity is a function of the square of the field strength. FIG. 6 is a diagrammatic graph of the intensity of the radiation utilized to pattern the feature centered around location 83. Since the intensity is the square of the field strength values, the sidelobes have a positive value, as does the primary lobe at location 83. Accordingly, if the magnitude of sidelobes 88 is sufficient, the sidelobes can generate printed features in the radiation-sensitive material 80. However, the magnitude of the sidelobes generated from light passing through a single window is typically too small to form a printed feature.
FIG. 7 diagrammatically illustrates the additive effect of a sidelobe formed from radiation centered around location 83, and a sidelobe formed from radiation center on location 85. The radiation formed around location 85 is identical to that formed around location 83, and accordingly a main lobe of radiation occurs at location 85 which is about identical in magnitude to the main lobe occurring around location 83. However, since a sidelobe from the radiation around location 85 overlaps a sidelobe from around location 83, the two sidelobes combined to form a lobe 94 having increased intensity relative to the main lobes at locations 83 and 85. Lobe 94 results from combination of sidelobes from two adjacent patterns of radiation, and the concept illustrated in FIG. 7 can be extended to combinations of three or more proximate patterns of radiation. Accordingly, a lobe formed from the combination of the sidelobes can grow significantly in intensity relative to the main lobes occurring at the centered regions of the patterns, and eventually the lobe formed from the combined sidelobes can have sufficient intensity to generate a printed feature.
Various techniques have been developed for identifying locations where sidelobes may combine to form a printed feature. Once such locations are identified, a radiation-patterning tool can be modified to prevent the undesired combination of sidelobes. For instance, FIG. 8 illustrates a portion of a mathematical construct 98 utilized to form radiation-patterning tool 84 (FIG. 4). Construct 98 comprises comprising four design features 101 which correspond to the elements 86 of radiation-patterning tool 84, and illustrates a prior art method for identifying a location where sidelobes may combine during utilization of the radiation-patterning tool. Construct 98 may correspond to a mathematical model formed at step 10 of the FIG. 1 process.
A calculation is performed on the spatial characteristics of design features 101, and such calculation can be considered to create a polygon 100 extending between the features 101. It is to be understood that the calculation is occurring in a mathematical domain during development of a pattern for a radiation-patterning tool, and accordingly the polygon 100 is not a real feature. The shown polygon is a rectangle extending from vertices of adjacent elements. The rectangle is subsequently utilized to determine the location of a sidelobe inhibitor 102. The location is initially determined as part of the mathematical model, but eventually the location is shifted to the real domain and an actual sidelobe inhibitor is formed at a real location of a radiation-patterning tool corresponding to the location 102 of the mathematical domain. The sidelobe inhibitor is utilized to prevent formation of an undesired printed feature from occurring at the location where sidelobes from radiation passing through the elements 86 (FIG. 4) converge.
FIG. 9 shows a portion of a radiation-patterning tool 84 identical to the tool described with reference to FIG. 4, except that a sidelobe inhibitor 103 is formed to prevent sidelobes of radiation passing through windows 86 from combining. Inhibitor 103 has length and width dimensions of “X” and “Y”. Such dimensions will typically be about ½ of the wavelength of radiation passed through tool 84 to form the pattern in the radiation-sensitive material. Inhibitor 103 can be formed by etching into an opaque material associated with tool 84 to form a region where radiation will be in phase with the main lobe of FIG. 5, and thus out of phase relative to other portions of sidelobe radiation. Such can cause destructive interference which ultimately cancels a significant amount of intensity from the combined sidelobes. The inhibitor 103 can thus correspond to a phasing region.
FIG. 10 is a color representation of a construct 97 showing actual results obtained from a prior art routine utilized to calculate placement locations for sidelobe inhibitors. Construct 97 corresponds to a mathematical model describing locations for placements of elements (shown as blue squares) and sidewall inhibitors (red blocks). Construct 97 also contains the polygons generated by the sidelobe placement routine (brown lines). The calculations utilized for determining the location of the sidelobe inhibitors would typically occur between steps 10 and 20 of the FIG. 1 process. In other words, the calculations would occur after a rough mathematical model is created for placement of elements in a radiation-patterning tool, and before optical proximity correction. The elements are actually represented as spatial information (also referred to herein as design features) in the mathematical model.
The placement routine described with reference to FIG. 10 will occasionally misidentify a location where sidelobe interference occurs. This can lead to placement of a sidelobe inhibitor at the wrong location. Also, the methodology described with reference to FIG. 10 will occasionally not recognize multiple discrete locations where sidelobe interference will occur if the locations are too close to one another. Such can result in a single sidelobe inhibitor being placed across an average of several locations where sidelobe combinations occur, rather than being placed at each discrete location where the combinations occur. This can result in some locations not having an appropriate sidelobe inhibitor provided.
It would be desirable to develop new methodology for identifying locations of radiation-patterning tools where sidelobe inhibitors are to be provided.